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Crossing relationships were studies within and among the variable populations of Pinus muricata to test hypotheses about crossing barriers among certain populations. Crossability was assessed at the level of viable seed production following planned crosses. Populations north of Sea Ranch, Sonoma Co., California, crossed freely with parapatric but genetically distinct populations in central Sonoma Co., although some reduction in seed-set occurred in the F2 and backcrosses to F1. The distinctness of these adjacent poulations is most likely not maintained by post-pollination crossing barriers. Crossability of disjunct P. muricata populations generally decreased with distance between populations. populations north of Sea Ranch crossed freely with the Pt. Reyes poulation in Marin Co., less readily with the Monterey population, and not at all with the Purisima (southern California) or Baja California populations. Mainland and island P. muricata populations south of Monterey were highly interfertile. Test crosses were also attempted between P. muricata and the island populations of P. radiata,which have been considered closely related to southern P. muricata populations. Pinus muricata from Baja California did not cross, however, with either Guadalupe Island pine (P. radiata var. binata) or Cedros Island pine (P. radiata var. cedrosensis). Together with results from other crossing studies in the Californian closed-cone pines, the patterns of crossability indicate three crossing units in P. muricata: 1)northern P. muricata populations from Marin Co. northward, which are reproductively isolated from, 2)southern P. muricata populations including mainland and Channel Island populations from Purisima southward, and 3)Monterey P. muricata, which is intermediate between the first two units.